A photosensitive resin composition is applied on a substrate, and forms a coating film. The entire coating film is exposed, so that an insulation film or a passivation film is formed. Alternatively, after a specific portion of the coating film is covered with a photomask and then exposed using light, unexposed portions of the coating film are removed by development. In this way, the photosensitive resin composition may be used to form a pattern.
Since being capable of being polymerized and cured by the radiation of light, the photosensitive resin composition is used for a photocurable ink, a photosensitive printing plate, various photoresists, a color filter photoresist for a LCD (Liquid Crystal Display), a photoresist for a resin black matrix, a transparent photosensitive material, or the like.
Among them, the transparent photosensitive resin composition is used for a column spacer, an overcoat, and a passivation film. In general, the transparent photosensitive resin composition means liquid composition that does not use coloring matters such as pigments and includes an alkali soluble resin, a polymerizable compound having an ethylenically unsaturated bond, a photopolymerization initiator, and a solvent. As the LCD has been used for quality enhancement and diversification, the LCD has been manufactured for a liquid crystal display device, such as a TV and a monitor, other than a laptop computer and a mobile device. Further, there has been a demand for a material that quickly reacts to light and has excellent mechanical properties in order to improve productivity and durability. When a pattern is formed by photo-lithography or an insulating passivation film is formed by exposing the entire surface, a property quickly reacting to light, that is, photosensitivity is a very important factor. Further, when a pattern is formed, portions not reacting to light should completely removed not to pollute a liquid crystal layer or cause troubles in the next process. Furthermore, the mechanical property of a column spacer pattern serving as a support should be excellent so that the liquid crystal display device is not damaged by an impact applied from the outside and normally functions. This demand can be achieved by the increase of an upper area of the pattern.
An alkali soluble resin functions as a binder matrix in a photosensitive resin composition, and the amount thereof to be added is largest. The structure of the alkali soluble resin should be optimized according to the purpose thereof so that the portions not reacting to light are removed as soon as possible by an alkali aqueous solution and quickly react to light, and distortion is minimized due to plastic processing in which heat is applied. When a column spacer is formed on the overcoat thin film, the property completely removed by the alkali aqueous solution becomes particularly important. In general, the overcoat is an organic thin film that is formed by crosslinking a resin composition with light or heat. For this reason, the overcoat has large affinity for a polymer of the column spacer and a cross-linker in comparison with an Indium-Tin Oxide that is an inorganic thin film. Therefore, when the column spacer resin composition not reacting to light is removed by an alkali aqueous solution, the column spacer resin composition may not be completely removed and remain on the overcoat in the form of a small grain. After that, when an alignment layer is processed in a mechanical rubbing process so that liquid crystal is aligned, the column spacer resin composition may cause scratches as a foreign substance. Further, when a crosslinkage is quickly performed due to excellent reactivity to light, the structure of the crosslinked polymer becomes denser. As a result, it is possible to obtain advantages of decreasing the pollution sources of liquid crystal, increasing a residual film ratio of the pattern, and increasing availability of other raw materials in the manufacture of a composition. The small thermal deformation during plastic processing is closely linked to the apparent strength of the pattern. The thickness of a column spacer is reduced and the shape of an upper portion thereof is changed after the plastic processing in comparison with before the plastic processing. If the change is large, the area of the portion supporting a thin film transistor substrate is decreased and the resistance of a panel against an external force is decreased. Accordingly, maintaining the area of the portion supporting a thin film transistor substrate by reducing the change in the plastic processing is closely linked to maintaining the mechanical properties of the column spacer.
In order to achieve these characteristics, when an alkali soluble resin is formed, various functional monomers need to be contained by the copolymerization so that the structure thereof is optimized. It is effective that the appropriate amount of the monomers, which have advantages corresponding to the above-mentioned characteristics, is added and copolymerized. If the molecular weight and the acid value of the polymer obtained in this way are adjusted, it is possible to obtain desired effects without separate additives.
The following polymer resin has been used in a transparent photosensitive resin composition known until now. In the polymer resin, a methyl methacrylic acid containing a carboxylic acid so as to be removed by alkali and a copolymer of benzyl methacrylate giving adhesive strength to the pattern are used as base materials, and various functional monomers are added to the base materials. For example, the following method has been disclosed in Korean patent application publication No. 2001-0018075. In the method, a self-curable material is added to the polymer resin having the above-mentioned structure, so that a chemical bond is reinforced at a portion crosslinked by light. Accordingly, the difference in solubility is improved, the resolution is increased, and the amount of a crosslinked compound to be used is reduced. However, according to this method, it is not possible to improve the developing property of a portion that is not exposed by light. Therefore, there are disadvantages in that the pollution of liquid crystal or problems caused by a mechanical rubbing process for aligning liquid crystal cannot be solved. An additive including an organic acid such as a carboxylic acid may be used to minimize the disadvantages. However, there are possibilities in that the additive is expensive or has low solubility and becomes a pollution source of liquid crystal. Due to the possibilities, the additive is limitedly used.
For this reason, it is not possible to obtain desired effects, and a polymer resin capable of minimizing accompanying disadvantages needs to be developed.